The present invention relates to an electrolytic reduction pot for the production of aluminum by fused salt electrolysis, said pot comprising an outer steel shell, thermal insulation and an inner lining essentially of carbon with iron cathode bars embedded in it, the floor insulation at least in part comprising a layer of mechanically compacted granular material of ground insulation layers and having essentially a particle size that varies between 0.01 and 8 mm.
For the production of aluminum by fused salt electrolytic reduction of aluminum oxide the latter is dissolved in a fluoride melt made up for the greater part of cryolite. The cathodically precipitated aluminum collects under the fluoride melt on the carbon floor of the cell where the surface of the molten aluminum forms the actual cathode. Dipping into the melt from above are anodes which in conventional processes are made of amorphous carbon. At the anodes, as a result of the electrolytic decomposition of the aluminum oxide, oxygen is produced which reacts with the carbon of the anodes to form CO.sub.2 and CO. The electrolytic process takes place in a temperature range of approximately 940.degree.-970.degree. C.
The electrical energy consumed in the electrolytic process can be classified in two main categories:
Production or reduction energy PA1 energy losses.
The productive part of the energy that is consumed is required in order to reduce the Al.sup.3+ cations to metallic aluminum. This productive part of the energy consumed can therefore not be lessened.
The energy losses on the other hand can be divided into various components all of which have the effect of dissipating heat losses to the surroundings. The heat produced in the electrolytic process always flows to the colder part of the pot; from there it escapes to the surroundings thus removing energy from the production process. These heat losses can be checked and must be brought to a minimum.
By using optimally suited materials for the electrical conductors the voltage drop and with that the energy losses in the electrical circuit can be reduced to a minimum.
For a long time now it has been customary to provide a thermally insulating layer in the outer steel shell in order to prevent the loss of heat through the pot or to reduce this to a low level. Usually brick made of diatomaceous earth or moler stone is employed. New moler stone materials have excellent insulating properties; they are however very sensitive to components of the electrolyte bath which penetrate the carbon lining. For this reason the insulating layer lying closest to the electrolyte bath is often made out of less temperature sensitive, electrolyte resistant, but poorer insulating firebrick. As such bricks can be readily stacked on top of each other, it is possible to insulate the sidewalls and the floor of the pot without any difficulty.
Proposed in the U.S. Pat. No. 4,052,288 is to grind the linings of spent reduction cells i.e. residual carbon and insulation, and then to treat this with a strong alkaline solution so that the fluorides of sodium and aluminum are removed. A binder, usually petroleum pitch, is then added to the filtrate to produce a paste for lining new reduction cells.
Described in the U.S. Pat. No. 4,430,187 is a reduction pot in which at least the lower 80% of the cell floor insulation is made up of a compacted vulcanic ash layer, the rest of the insulation on the cell floor of a leakage barrier which screens the vulcanic ash from the bath components penetrating the carbon lining.
Known from the U.S. Pat. No. 4,548,692 is that at least the lower 75% of the floor insulation can be of a mechanically compacted layer of a granular material having a particle size ranging essentially from 0 to 8 mm. This granular material contains the fully ground, but otherwise untreated insulation layers, without residual carbon which is mechanically sorted out before grinding, from scrapped electrolytic cells. The remaining 0-25% of the floor insulation is made up of a layer of firebrick, ground firebrick and/or smelter alumina. The sidewalls of the steel shell are insulated solely by firebrick.